Even though cosmetic preparations have been a staple of commerce for centuries, there continues to be a continual expansion of cosmetic markets, and proliferation of new products. The field of skin care products has grown tremendously as new naturally occurring and synthetic polymeric materials have been identified. Processing equipment and techniques have also become more sophisticated, so that many of the current cosmetic products available over the counter, have a high level of uniformity and wholesomeness, but also impart substantive pharmacologic and nutritive properties not realized in older preparations.
The goal of modern cosmetics is to achieve multiple effects simultaneously, such as moisturizing, increased pliancy, drug or nutrient delivery, texturizing, environmental protection, and coloration. Some of these objectives inherently require substances which are water compatible such as humectants, and others require lipid compatible substances such as emollients. The largest class of cosmetic mixtures are emulsions of oil and water based fractions, and may be either oil in water (O/W) or water in oil (W/O), or both. For a review of the various physical forms and processes of making mixtures such as true emulsions, microemulsions, gels, and liposomes, see Fox, C., "Cosmetic Vehicles., Advances in Cosmetic Science and Technology, part 4," Cosmetics and Toiletries, 110: 59 (1995).
These emulsions have both occlusivity and humectancy. In occlusivity, a non-irritating moisture barrier is established on the surface of the skin, to prevent loss of moisture. Typical materials are petroleum jelly, mineral or vegetable oils, silicones, waxes, fatty acids, and esters. Typical humectants, which attract water and help bind water to the skin, include glycerin, sorbitol, sodium lactate, and sodium pyrollidone carboxylate, are beneficial in hydrating the stratum corneum and improving its viscoeleastic behavior (See Williams and Schmitt, eds., Chemistry and Technology of the Cosmetics and Toiletries Industry, 2nd ed., Blackie Academic and Professional, London: 1996).
Most emulsions require one or more stabilizers to maintain the microdispersion, and to prevent collapse and separation of the blend. A widely used stabilizer system utilizes stearic acid in the oil phase, and tri-, di-, or monoethanolamine in the aqueous phase. The resulting ethanolamine stearate is an excellent stabilizer. Combinations of intermediate chain length (C6-C12) aliphatic alcohols, esters, and carboxylic acids may act as stabilizers. Cetyl alcohol (C16) is commonly used in conjunction with triethanolamine to impart both soft "feel" and stabilization of the emulsion. Cetyl palmitate, isolated from natural sources, is a preferred stabilizer and carrier for other ingredients.
Many cosmetic preparations require thickeners to provide the desired viscosity. These thickeners are often used when the lubricity of the oil fraction is heightened by a high unsaturated oil composition of the cosmetic. Typical thickeners include coconut diethanolamide, cellulose and certain ether derivatives such as hydroxyethylcellulose or hydroxypropylcellulose, carrageenan, which is a linear sulfated polysaccharide of D-galactose and D-anhydrogo-D-galactose. For a good discussion of the role of thickeners in cosmetic preparation, see Knowlton, et al., eds., Handbook of Cosmetic Science and Technology, 1st ed., Elsevier, Oxford: 1993).
Since the advent of polymer surfactant chemistry, a large and varied number of surface active compounds have been available for incorporation into cosmetics. Because surfactants tend to mobilize natural lipid, there is a pronounced tightening effect on the skin associated with their use, presumably by their residues binding to keratin. This effect is most severe using sodium lauryl sulfate, and is much diminished in the use of potassium myristate. The use of surfactants, of course, has an important cleaning function, so that they are common ingredients in cleansing creams. Since these creams are first spread onto the skin and then removed almost immediately by tissue wiping, surface contact is minimized.
There have been many studies of cosmetic additives which enhance the penetration of the beneficial ingredients into the lower strata of the epidermis. Similarly there have been many studies of the physiology, histology, and biochemistry of the epidermis and dermis and their relation to absorption. Among those compounds studied, the following have been identified as promoting adsorption and penetration (also termed "flux") of chemicals through the lipid-dominated interstices between cells of the lamellar layer: propylene glycol either alone or in combination with unsaturated fatty acids, and propylene glycol diesters of caprylic and capric acids. Solvents such as DMSO, ethanol, hihydrotetrafuran, and isopropanol are known to deplete interstitial lipids leading to increased penetration of indicator drugs. These may be useful for "patch" type drug delivery devices, but have limited value in routine cosmetics because of their irritant properties. The goal of these ingredients is to promote adsorption and migration of beneficial substances through the lipid deposits without disrupting the essential structure of the skin.
A popular class of cosmetic additives (15 percent of total cosmetic sales) is the so-called sunscreens. UV light from the sum has a spectrum of wavelengths ranging from 400 nm down to less than 290 nm. Light having a wavelength of 400-320 is known as UV-A, that having a wavelength of 320-290 is called UV-B, and that having a wavelength of less than 290 nm is UV-C. UV-B has the greatest potential for causing skin damage and is associated with accelerating skin ageing, wrinkling, epidermal cracking and scaling, and more serious conditions such as basal cell carcinoma and melanoma. UV-A is associated with melanin production resulting in tanning. The desired function of a sunscreen is to absorb out the harmful UV-B light while allowing the UV- light to effect tanning. Ideally a sunscreen will shield the skin from a portion of the UV-A light as well as the UV-B because overexposure to UV-A can also cause skin damage.
Conventional sunscreen compounds that absorb UV-B light include benzophenone-3, benzophenone -4, and benzophenone-8, 4-methoxycinnamic acid salts, parabenzoic acid, substituted parabenzoic acid and salts thereof, glyceryl parabenzoic acid, menthyl anthranilate, TiO.sub.2 and ZnO. UV-A is effectively absorbed by butyl methoxydibenzoyl methane. While most sunscreen compounds are spread onto skin in liquid form, dry compositions have been disclosed in U.S. Pat. No. 5,676,934. Other sunscreen formulations of interest are the stabilized preparations disclosed in U.S. Pat. Nos. 5,670,139, 5,573,755 (hydroxy salts of metallic ions), U.S. Pat. No. 5,543,136 (conventional titanium and zinc oxides in a smooth, even flowing emulsion stabilized with tridecyl polymers). Finally, certain compounds have been shown to enhance the activity and effectiveness of conventional classes of sunscreen compounds, as disclosed in U.S. Pat. No. 5,607,664.
In the formulation of cosmetic preparations, most of the additive ingredients, e.g. humectants, sunscreens, surfactants, thickeners, etc. are present in low concentrations ranging from 0.1 to about 10 percent. Sunscreen concentrations are illustrative, and are also regulated by domestic and foreign governmental agencies, as set forth in the Cosmetic Handbook, supra: bendzophenone-3 (2-6%), benzophenone-4 (5-10%), benzophenone-8 (3%), 4-methoxycinnimic acid salts (8-10%), parabenzoic acid (1-5%), and butyl methoxydibenzoyl methane (5% maximum). The major difference in formulary is whether the emulsion is an O/W or a W/O. Obviously there will be a widely differing overall oil content in a W/O preparation. In the case of a high oil content all purpose cream, a typical will have sensory components (stearic acid, cetyl alcohol, microcrystalline wax, glyceryl monostearate and the like in a concentration of 0.5 to 3 percent, but the combination of mineral oil and petrolatum will approach 50 percent. In contrast, a hydroalcoholic astringent will contain nearly 50 percent ethyl alcohol, witch hazel extract, and glycerine. In a low oil formulation, the oil content may be only a few percent, and the texture of the preparation will rely upon thickeners and emulsion stabilizers such as triethanolamine in combination with stearic acid.
There have been many variations of cosmetic preparations incorporating skin active agents in delivery systems of varying efficiency. U.S. Pat. No. 5,618,850 discloses a preparation incorporating alpha-hydroxy acid salts such as lactate and glyconate. The alpha-hydroxyacids have attracted much attention for their ability to control skin texture and wrinkling, and to confer beneficial effects in treating mild skin conditions. U.S. Pat. No. 4,393,043 is interesting in that it discloses the enrichment of cosmetic preparations with essential fatty acids including linoleic acid and conjugated linoleic acids, but only at the levels found in jojoba oil. In a similar disclosure in U.S. Pat. No. 4,661,343 an essentially anhydrous preparation utilizes karite oil to impart a mixture of fatty acids including linoleic acid in a concentration of 2 to 5 percent.